When a laptop, tablet or device housing must be thinner and stiffer but still look flawless after anodising, the alloy choice becomes a trade-off between strength, formability and cosmetic quality. Higher-strength 5xxx alloys suit stamped cosmetic shells, heat-treatable 6xxx alloys suit machined housings that also carry load and heat, and low-copper 7xxx alloys give the highest strength where finish can be carefully controlled. This guide explains each route and how to specify anodising-quality sheet; Metline supplies the aluminium sheet, plate and coil worldwide.
Anodising does not hide the metal underneath; it makes the oxide out of it. Every particle, alloying element and grain in the surface layer shows up in the colour and gloss of the finished part. The elements and microstructures that make aluminium strong are often the same ones that make an anodised surface uneven.
Copper, the main strengthener in 2xxx alloys and an important addition to many 6xxx and 7xxx alloys, tints the anodic film yellow or grey and makes dyeing less consistent. Silicon and iron form hard intermetallic particles that appear as dark specks or a slightly hazy surface. Coarse or uneven grains, and the texture left by rolling or extrusion, appear as streaks, bands or an "orange-peel" pattern after brightening. Higher magnesium raises strength in 5xxx alloys, but if the metal is not clean and fine-grained it can also give a duller finish.
Thin-wall design makes all of this harder. A housing that drops from 1.0 mm to 0.7 mm must be stronger to keep the same dent resistance, but the cosmetic surface is unchanged, so customers notice any defect. The practical goal is not the strongest alloy, but the strongest alloy that still passes the cosmetic inspection reliably at production volume.
Non-heat-treatable Al-Mg sheet in harder tempers. 5052 is the baseline; 5252, with lower iron and silicon, gives a brighter, more uniform anodised finish. Best for stamped and drawn cosmetic shells where forming matters as much as strength.
Al-Mg-Si alloys such as 6063, 6061 and the higher-strength 6013. Higher yield strength after ageing, good thermal conductivity and excellent machinability. Best for CNC unibody housings, frames and parts that carry load or spread heat.
Al-Zn-Mg alloys such as 7003 and 7021, whose copper is held to a few tenths of a percent, give much higher strength than 6xxx while anodising far better than copper-rich 7075. Best for thin, stiff frames and premium shells, with tighter process control.
Each route has a different weak point. 5xxx alloys cannot be heat treated, so strength comes only from cold work and is limited. 6xxx alloys need solution treatment and ageing, and stamping is usually done in a softer temper before ageing. 7xxx alloys are the strongest but are more sensitive to grain structure and to colour variation from zinc and copper, and they are less formable at room temperature.
Values are typical figures published by the Aluminum Association and ASM for sheet or plate. They show relative strength; specified minimums from ASTM B209 or EN 485-2 apply to orders and vary with thickness.
| Alloy and temper | Typical tensile | Typical yield | Anodised appearance | Main processing route |
|---|---|---|---|---|
| 5052-H32 | 228 MPa | 193 MPa | Good; slight grey tone | Stamping, bending |
| 5052-H38 | 290 MPa | 255 MPa | Good | Light forming, flat parts |
| 5252-H25 | 235 MPa | 170 MPa | Very bright and uniform | Stamping, bright trim |
| 6063-T6 | 241 MPa | 214 MPa | Excellent, very uniform | Extrusion, machining |
| 6061-T6 | 310 MPa | 276 MPa | Very good | CNC machining from plate |
| 6013-T6 | Higher than 6061-T6 | Higher than 6061-T6 | Good; copper adds slight tint | Machining, formed in T4 then aged |
| 7003 / 7021 (aged) | Well above 6xxx | Well above 6xxx | Good with controlled grain | Machining, extrusion |
| 7075-T6 | 572 MPa | 503 MPa | Fair; yellowish, less even | Machining; rarely for cosmetic faces |
Where a supplier quotes a proprietary or in-house grade code for "high-strength anodising" sheet, ask for the public alloy designation, the guaranteed minimum properties and an anodised sample before approving it.
| Part type | Priorities | Usual route |
|---|---|---|
| Stamped laptop lid or base shell | Formability, cosmetic yield, moderate strength | 5052 or 5252 in H32 – H38; 6xxx if more stiffness needed |
| CNC unibody housing | Strength, flatness, machinability, finish | 6061-T6 / T651 or 6063-T6; 6013 for a strength upgrade |
| Thin structural frame or middle frame | Highest strength and stiffness at low wall | Low-copper 7xxx, or 6013 |
| Housing that spreads heat | Thermal conductivity plus strength | 6063 or 6061 (higher conductivity than 5xxx and 7xxx) |
| Internal brackets, shields | Formability, cost; cosmetics less critical | 5052, 3003, or 6061 |
| Bright trim, bezels, buttons | Mirror-bright anodised finish | 5252, 6063, or bright-trim 5xxx |
A useful rule: start from the conventional alloy for the process (5052 for stamping, 6061 or 6063 for machining), and move up to 5252, 6013 or 7xxx only when the thinner wall genuinely needs more strength or the finish needs to be brighter. Every step up adds cost and narrows the process window.
Standard commercial sheet is made to mechanical and dimensional standards, not to cosmetic anodising requirements. For housings, the order must add cosmetic conditions explicitly.
| Defect after anodising | Likely material cause | Prevention |
|---|---|---|
| Streaks or bands along the rolling direction | Non-uniform grain or texture, segregation | Anodising-quality sheet with controlled grain; consistent rolling direction on parts |
| Dark specks or haze | Iron- and silicon-rich intermetallic particles | Lower Fe and Si; brighter alloys such as 5252 |
| Yellowish or grey tint | Copper or high zinc content | Low-copper alloys; avoid 2xxx and 7075 on cosmetic faces |
| Orange peel after forming | Coarse grains stretched at the surface | Fine-grained material; avoid excessive strain |
| Colour difference between parts | Different casts, coils or tempers | Lot control and approved sample panels |
| Pits after etching | Inclusions or surface contamination | Clean metal, protective film, careful handling |
Many defects are blamed on the anodiser but start in the metal. Equally, a perfect sheet can be spoiled by over-etching or inconsistent bath control, so material and finishing trials should be run together.
Use the hardest temper that still forms the part; tighter radii crack the oxide later. Keep the film on during forming.
Use sharp tools and consistent finishing passes; tool marks and burnishing show through clear anodising.
For 6xxx and 7xxx parts formed in a soft temper, age after forming and before final machining of cosmetic faces.
Sandblasting, brushing or polishing must be uniform; variation becomes visible after anodising.
Type II sulphuric anodising is standard for colour; hard anodising is harder but darker and less decorative.
Proper sealing locks in dye and improves corrosion and fingerprint resistance.
6063 and bright-trim 5xxx alloys such as 5252 give the most uniform, bright anodised finish. 5052 and 6061 are also very good and stronger. Copper-rich alloys such as 2024 and 7075 give the poorest cosmetic results.
It can be anodised, but the finish tends to be yellowish and less uniform. Low-copper 7xxx alloys such as 7003 or 7021 are usually preferred when 7xxx strength is needed on a cosmetic surface.
5052 suits stamped shells because it forms well in H32 – H38 tempers. 6061-T6 suits CNC unibody housings because it is stronger and machines cleanly. Many designs use both for different parts.
Sheet produced and inspected for cosmetic anodising: controlled chemistry, fine and uniform grain, a clean surface and freedom from streaks. It must be specified on the order, because standard sheet is not tested for appearance.
Small differences in chemistry, grain structure or temper between casts or coils change how the oxide grows and takes dye. Keep builds on one lot and approve colour against reference panels.
No. Metline supplies the aluminium sheet, plate, coil and extrusions, and can cut blanks to size; the housings are stamped, machined and anodised by the device maker or its suppliers.
Send the part type, target wall thickness and cosmetic finish. Metline will suggest suitable grades and reply with availability and a delivered price.
